The present invention relates to apparatus for detecting particles in an aerosol by light scattering and, more particularly, to a system employing an integrating sphere for detecting and measuring particles as small as submicron size in aerosols.
Particles in aerosols have been detected by detecting light scattered by the particles as they pass through a beam of light, such as a laser beam. In prior art systems, the scattered light is collected by a lens or lenses and focused on a photodetector. The amplitude of each resulting pulse generated by the photodetector in response to a particle indicates particle size. When an elongate particle scatters light in such a system, light is scattered with different intensities in different directions. As a result, the pulse amplitude will depend upon the particle orientation as it passes through the laser beam. In addition, the lenses of such systems of the prior art are subject to contamination, and, in the manufacture of such a system, the lens or lenses have to be carefully aligned with the photodetector and the locus in which the particle encounters the laser beam.
An integrating sphere has been used in connection with a laser beam in a turbidimeter to measure the turbidity of a sample liquid by optical diffusion where the sample liquid contains dispersed particles of submicron size in an extremely low concentration. The liquid sample is guided through the interior of the integrating sphere by a cylindrical sample cell, and a collimated beam of light is projected axially through the cell and the sample therein.